One of the most contemporary challenges to the treatment of hospital-acquired infections worldwide is the appearance and global spread of staphylococci resistant to all ?-lactam antibiotics (known as methicillinresistant Staphylococcus aureus; MRSA). More recently, MRSA has also become established outside of the hospital, appearing in community populations without healthcare association or identifiable risk factors for infection. In either case, resistance to ?-lactam antibiotics is due to the acquisition of a gene (mecA) that encodes a ?-lactam insensitive target enzyme, penicillin-binding protein (PBP)2a. This enzyme affords the bacterium the ability to cross-link cell wall and grow while the cell's usual cross-linking enzymes are bound and inactivated by ?-lactam antibiotics. Several studies have shown that the morbidity and mortality in infections due to MRSA are considerably higher than in MSSA (methicillin susceptible Staphylococcus aureus) infections, mainly because of an inadequate initial antimicrobial therapy. As a consequence, accurate detection of methicillin resistance in S. aureus is not only clinically important but essential for hospital infection control programs. ?-lactam antibiotics are agents of choice to treat (MSSA) because of their bactericidal activity. The main characteristic of MRSA strains is their heterogeneous expression of ?-lactam resistance. We have examined a number of clinical MRSA strains that are misinterpreted as MSSA due to their extreme heterogeneity. These isolates were referred to our laboratory from the Centers for Diseases Control and Prevention (CDC, Atlanta, GA) as clinically relevant since exposure of such isolates to ?-lactams can result in high-level resistance (HoR). Indeed, we have determined in a representative strain SA13011, that selection from heterotypic ((HeR) to homotypic resistant (HoR) phenotype occurred after exposure to sub-inhibitory concentrations of ?-lactam antibiotics. This selection involved, in addition to increasing expression of MecA, the triggering of ?-lactam-mediated SOS response and increased mutation rate. Therefore, the central hypothesis of this proposal is that upon exposure to ?-lactams, SA13011 is selected from heterotypic (HeR) to homotypic resistant phenotype (HoR) by a ?-lactam-induced SOS response that leads to an agr-genetically controlled increased mutation rate that helps to maintain, among others, the cell wall integrity. SA# 1.To define the role of ?-lactam induced SOS response on the development of high level resistance to oxacillin in the clinical methicillin- resistant isolate SA13011. SA# 2. To identify the mechanisms involved in ?-lactam-induced SOS response induction and cell wall integrity during oxacillin-mediated HeR-HoR selection SA #3. To investigate the regulatory role of agr in ?-lactam-induced SOS- response and mutation rate.